Moisture sensing control



Sept. 13, 1966 R. G. MARTIN MOISTURE SENSING CONTROL Filed Oct. 14, 1963Y R m M N e E a V4 7 WM 4 6 0W M United States Patent This inventionrelates to dryer controls of the direct moisture sensing ty-pe.

Such controls operate on the basis of direct sensing of moisture contentby monitoring the resistance of the mate-rial being dried which willvary in accordance with the moisture content of the material. Thecontrol senses to a predetermined moisture content at which point itshuts down the dryer, either directly or with an intervening cool-downcycle, or alternatively can be used to initiate a timed drying cycleafter the direct sensing cycle. Examples of dryer controls of this typecan be found in the following US. patent applications which are assignedtothe assignee of this application: Dale F. Willcox application forDryer Control, Serial No. 31,922, filed May 26,1960, now abandoned; OttoH. Behrens application for Control Device, Serial No. 45,329, filed July26, 1960; and Robert F. Guenther, Jr., and Ronald G. Martin for ControlDevice and Timer, Serial No. 243,001, filed December 7, 1962.

A general object of this invention is to provide an improved version ofthis type of control.

A more specific object of this invention is to provide a direct moisturesensing control which achieves accurate control to low moisture contentswith virtually no mechanical moving parts required thereby eliminatingcalibration problems generally encountered in mechanicaltype controlsand reducing the risk of failure.

Another object of this invention is to provide a completely electronicmoisture sensing control which is effective to accurately control dryeroperation to low values of moisture retention and compensate for dryingmixed loads; and, furthermore, to reduce fluctuations in controloperation due to variations in input voltage by providing regulation ofthe input voltage.

Other objects and advantages will be pointed out in, or be apparentfrom, the specification and claims, as will obvious modifications of theembodiment shown in the drawings which illustrates a portion of thecircuit of a dryer including a moisture sensing control constructed inaccordance with this invention.

With particular reference to the drawings, a dryer drum ltlis providedwith a direct sensing mechanism which, in this instance, takes the formof a pair of sensing rings 12 and 14. Sensing ring 12 is connected toground and sensing ring 14 is connected in a control circuit in such amanner that a voltage is applied across the rings, this will bedescribed more completely hereinafter. The sensing rings are arrangedfor contact with and to be bridged by the material being dried so thatcurrent leakage between the rings occurs through and in accordance withthe moisture content of that material.

The electric circuit of the dryer can be considered in two parts, amoisture sensing and control portion 16 connected on the output side oftransformer 20 and an operational portion 18 connected on the input sideof the transformer.

Turning first to operational circuit 18, leads L and L are connected toa suitable source (not shown) of alter- .nating current and dryeroperation is initiated through switch 22. Operation of switch 22completes a circuit to and energizes relay 24 to close relay switch 26and complete a circuit to drive motor 28 of the dryer. Initially thecircuit to motor 28 is completed through switch 3,271,878 Patented Sept.13, 1966 22 and a centrifugal switch 30 so that both run winding 32 andstart winding 34 are in circuit. When the motor comes up to speed,switch 30 operates to move switch blade 25 from contact 31 to contact 33to complete a circuit to the motor through relay switch 26, door switch35 and contact 33 to thereby remove start winding 34 from the circuit.

It will be noted that primary winding 36 of transformer 20 is includedin operational circuit 18 whereas secondary winding 38 is connected insensing circuit 16 so that the output of the transformer is the sourceof electrical energy for the sensing circuit, with this arrangementtransformer 20 also operates as an isolation transformer.

Heater 29 of the dryer is also activated upon energization of relay 24through closure of relay switch 27. Electrical heat supply means isillustrated merely by way of example and it will be appreciated thatthis invention is not limited to any particular type of heat supplymeans.

During dryer operation relay 40, included in sensing circuit 16,controls the state of energization of relay 24 to thereby control dryeroperation, namely motor 28 and heater 29. More particularly, switch 42is connected to and operated by relay 40 and is included in circuit withrelay 24 so that operation of relay 40 opens switch 42 to open thecircuit to relay 24 and open switches 27 and 26 to de-energize both thedryer motor and heater. The state of energization of relay 40 iscontrolled by a thyratron 46 and, as will be discussed more completelyhereinafter, thyratron 46 is initially, i.e. at the start of a dryingcycle, in a non-conductive state so that the circuit to relay 40 is openbut, when a predetermined moisture content of the material being driedis reached, it is rendered conductive to activate relay 40 and openswitch 42 to deenergize relay 24.

Thyratron 46 is included in electronic sensing circuit 16 and, as willbecome apparent from the following description, will be controlled inaccordance with the condition of the material in dryer 10 as it contactssensing rings 12 and 14. More particularly, the output of transformer 20is rectified by diode 48 so that direct current flows in the sensingcircuit. The direct current passes through resistance R and a selector50 which includes resistances R and R and a selector switch 52 and isrepresentative of control means which can be included to afford somechoice in dryer operation. With the particular selector illustrated DRYor DAMP DRY operation can be selected, i.e open switch 52 to place bothR and R in circuit to achieve a DRY operation or close the switch toremove R for a DAMP DRY operation. Sensing ring 14 is connected in thesensing circuit and will have the direct current impressed thereon and,ring 12 being connected to ground, the potential across the rings willvary in accordance with the current leakage through the material beingdried which will vary with the moisture content of the material, i.e. asthe material dries the resistance thereof will increase and thepotential on ring 14 will increase accordingly.

A neon lamp N is connected in circuit with sensing ring 14 and betweenit and thyratron 46 to act as a switching device which is effective toperiodically apply the charge on ring 14 to grid 46g of the thyratron.More particularly, neon lamp N is normally non-conductive when dryeroperation is initiated as its switch-over voltage is above the voltagewhich will appear on ring 14 at the start of the drying cycle due to thehigh rate of leakage which occurs through the material having arelatively high moisture content. Capacitor C is also connected in thecircuit with the neon lamp N and sensing ring 14 and begins to charge tothe value of the voltage on ring 14 and this charge increases until itreaches the switch-over voltage for lamp N whereupon the lamp breaksdown and begins to conduct with capacitor C discharging therethrough.Resistance R cooperates with capacitor C to provide a filter networkwhich reduces voltage spikes across the rings and prevents prematurebreakdown of lamp N, which may occur where a small or suspended loadcondition is present in the drum and an artificially high potentialappears for a short time across the sensing rings. The charge oncapacitor C is dissipated through lamp N and is applied to grid 46g ofthe thyratron.

Thyratron 46 exhibits a particular switch-over voltage at which it isrendered conductive and the switch-over voltage of N is selected suchthat it is below the switchover voltage of thyratron 46 so that thecharge necessary on capacitor C to breakdown lamp N is insufiicient totrigger the thyratron. A second capacitor C is connected in the circuitbetween lamp N and thyratron 46 and is characterized by having a chargestoring capacity which is at least equal to the switch-over voltage ofthyratron 46. Thus, when lamp N is initially rendered conductive thecharge which passes through the lamp is insufficient to trigger thethyratron and is received and stored on capacitor C The charge oncapacitor C is dissipated through lamp N and will eventually diminish toa value below the maintaining voltage for lamp N; at which point thelamp will be extinguished and remain in a non-conductive state until acharge equal to its switch-over voltage is again accumulated oncapacitor C This periodic discharging of capacitor C will continue untila charge corresponding to the switch-over voltage of thyratron 46accumulates on capacitor C whereupon the necessary voltage is impressedon grid 46g to initiate current flow between plate 46p and cathode 46kand complete a circuit through relay 40. Switching thyratron 46 fromnon-conductive to conductive to achieve the control operation of relay40 is preferred as it insures reliable, predictable and consistentoperating characteristics from the relay. Resistance R is provided inthe grid circuit to maintain the current flow to the grid at a safelevel.

In operation, when the dryer is initially energized, thyratron 46 isnon-conductive and relay 40 is in an unactuated condition. Selectormechanism 50 will have been operated to select either a DAMP DRY or DRYoperation. A DC. voltage is applied to ring 14 and as the moisturecontent of the material in the dryer is reduced the potential acrossrings 14 and 12 increases and a charge accumulates on capacitor C Thecharge on capacitor C will eventually equal the breakdown voltage forlamp N, at which point the lamp is rendered conductive and capacitor Cdischarges through the lamp. The switch-over voltage of lamp N is belowthe switch-over voltage of thyratron 46 and this charge is ineffectiveto render the thyratron conductive and is received and stored bycapacitor C When the charge on capacitor C diminishes below themaintaining voltage of lamp N the lamp is extinguished without havingrendered thyratron 46 conductive and the drying operation continues andthe sensing control also continues to sense the moisture content of thematerial. Again a charge accumulates on capacitor C in accordance withcurrent leakage across rings 14 and 12 until that charge again reachesthe breakdown voltage of lamp N whereupon capacitor C will againdischarge through the lamp. Lamp N is periodically rendered conductiveuntil the charge on capacitor C reaches the switch-over voltage ofthyratron 46g when this condition is reached the thyratron is triggeredand completes a. circuit through relay 40. Relay 40 opens switch 42 tode-encrgize relay 24 which in turn opens relay contacts 26 and 27 tode-energize the heater and motor. Capacitor C together with resistance Rprovides an RC time delay in the grid circuit of thyratron 46.

Thus, a time delay between the initial discharge of neon lamp N andoperation of the thyratron to deenergize the heater is provided througha completely electronic arrangement. With this arrangement, the initialresponse of lamp N, can be selected to occur at a predetermined voltagecorresponding to a particular, desired moisture content of the materialbeing dried. But, before the thyratron is triggered, the lamp N isextinguished and the direct sensing cycle of the sensing circuit isrepeated. As set forth in the application of Dale F. Willcox, Serial No.31,922, all fabrics, synthetic or natural, if initially at the samemoisture content will dry in the same length of time so that if thefabrics are reduced to a uniform moisture content they can be reduced toa lower moisture content if desired by a straight timed cycle. Theelectronic time delay circuit can provide a timed cycle from themoisture content condition indicated by the initial discharge of lamp Nand by proper selection of the circuit elements (lamp N capacitor C andthyratron 46) the desired time delay can be adjusted. However, anadvantage in the electronic time delay circuit of this invention is thatsubsequent to initial discharge of the lamp, and during the time delayperiod, the sensing circuit is re-established so that direct sensing ofthe material is repeated and continues until the desired moisturecontent is reached. A control of this type provides continuing moisturesensing and automatic adjustment to or accommodation of mixed loads and,furthermore, will achieve accurate control to a moisture content rangeof 0 to 1%. Moreover, an additional advantage lies in the elimination ofa mechanical timer thereby eliminating the problems of initialcalibration, continued adjustment and possible mechanical failures.

To insure that capacitor C is uncharged at the start of a drying cycle,the capacitor is connected to ground through a switch 54 which, whenclosed, discharges any residual capacitor charge to ground. For example,switch 54 can be controlled with, or be a part of, door switch 35 andwhen the dryer door is opened at the end of an operation any residualcharge left on capacitor C is discharged.

Sensing circuit 16 also includes a voltage regulating circuit to affordcontrol over the voltage applied to sensing ring 14. The regulatingcircuit includes neon lamps N N and N and resistance R; across lamp N toassist in starting that lamp. The operation of the regulating circuitshown is well known in the art and is purely illustrative as othercircuit arrangements could be used equally well. In general terms thecharacteristics of this particular circuit are such that with anincrease in input current and corresponding increase in current flowthrough lamps N N and N the maintaining voltage of the lamps remainsrelatively constant so that the DC. voltage between lamp N and groundremains constant with variations in input voltage.

Among the features included in operational circuit 16, and commonlyincluded in a dryer, is to provide fora cool-down cycle at thetermination of the drying cycle. More particularly, after thyratron 46has been actuated and relay 24 operated, thermostatic switch 44 can beclosed, in any suitable manner, to complete a circuit through motor 28with heater 29 de-energized to continue dryer operation until apredetermined temperature is reached in the dryer drum. At this reducedtemperature switch 44 opens the circuit to motor 28 to shut down thedryer. Another feature commonly incorporated in the dryer is an airfluff operation and to this end air fiuif switch 55 is included incircuit with heater 29 and can be opened to remove heater 29 and permitthe dryer to operate without heat.

Although but one embodiment of the present invention has beenillustrated and described, it will be apparent to those skilled in theart that various changes and modifications may be made therein withoutdeparting from the spirit of the invention or from the scope of theappended claims.

What I claim is:

1. A moisture sensing control for use with apparatus for drying materialand operative to control the heat supply means of said apparatus inaccordance with the moisture content of said material, said moisturesensing control comprising, in combination,

sensing means including an electrode arranged for engagement with saidmaterial,

means for applying a DC. voltage to said electrode withthe voltagethereon varying in accordance with current leakage through said materialso that the voltage on said electrode increases as the moisture contentof said material decreases,

switch means for controlling energization of said heat supplymeans andhaving a switch-over voltage at which its operational state is changedto de-energize said heat supply means,

a neon lamp connected in circuit between said electrode and said switchmeans and having a switchover voltage at which it is rendered conductiveand below which it is non-conductive, the switch-over voltage of saidneon lamp being below that of said switch means and the voltage on saidelectrode at the initiation of the drying operation being below theswitch-over voltage of said neon lamp so that said switch means iselectrically isolated from said electrode and is in a first operationalstate,

capacitive means connected in circuit with said neon lamp and electrodefor receiving and storing a charge from saidelectrode and characterizedby having a capacity at least equal to the switch-over voltage of saidneon lamp,

and capacitive means connected in circuit between said neon lamp andsaid switch means and characterized by having a capacity at least equalto the switchover voltage of said switch means.

2. The moisture sensing control of claim 1 including an inputtransformer connected in circuit with said sensing means and means forrectifying the AG. output of said transformer to DC.

3. A moisture sensing control for use with apparatus for drying materialand operative to control the heat supply means of said apparatus inaccordance with the moisture content of said material, said moisturesensing control comprising, in combination,

sensing means arranged in said apparatus for engagement with saidmaterial,

means for applying a DC. voltage to said sensing means with the voltageon said sensing means varying in accordance with current leakage throughsaid material,

first normally non-conductive switch means for controlling operation ofsaid heat supply means and characterized by having a predeterminedswitch-over voltage at which its operational state is changed,

second normally non-conducting switch means characterized by having aswitch-over voltage below the switch-over voltage of said firstswitching means and a maintaining voltage at which it is conductive andbelow which it is non-conductive, said second normally .noneconductingswitch means connected in circuit with said sensing means and said firstnonconducting switch means to normally electrically isolate said firstnon-conducting switch means from said sensing means,

charge storing means in circuit with said second switch means and saidsensing means and characterized by having a charge storing capacity atleast equal to the switch-over voltage of said second switch means sothat a charge is stored on said charge storing means in response to thevoltage build-up on said sensing means as the moisture content of saidclothing is reduced until the switch-over voltage of said second switchmeans is reached whereupon said charge storing means discharges throughsaid sec- 6 0nd switch means and falls below the maintaining voltage ofsaid second switch means which is again rendered non-conductive,

and charge storing means in circuit with said first and second switchmeans and having a charge storing capacity at least equal to theswitch-over voltage of said first switch means for receiving and storingthe charge discharged through said second switch means until theswitch-over voltage of said first switch means is reached whereupon saidfirst switch means is rendered conductive to de-energize said heatsupply means.

4. The moisture sensing control of claim 3 including voltage regulatingmeans in circuit with and controlling the voltage applied to saidsensing means to maintain a substantially uniform voltage thereon.

5. The moisture sensing control of claim 3 including means in circuitwith and operative to vary the voltage applied to said sensing means.

6. The moisture sensing control of claim 3 wherein said apparatusincludes a door switch connected in circuit with and operative toconnect said charge storing means in circuit with said first and secondswitch means to ground and remove any residual charge thereon prior toinitiation of operation of said apparatus.

7. A moisture sensing control for use with apparatus for varying themoisture content of material and comprising, in combination,

sensing means arranged in said apparatus for engagement with saidmaterial, means for applying a voltage to said sensing means with thevoltage on said sensing means varying in accordance with current leakagethrough said material,

first switch means characterized by having a predetermined switch-overvoltage at which its operational state is changed and operative toeffect a control function in said apparatus upon said change inoperational state thereof,

first charge storing means in circuit with said first switch means andcharacterized by having a charge storing capacity at least equal to theswitch-over voltage of said first switch means,

second charge storing means connected in circuit with said sensing meansfor response to the voltage thereon,

and second switch means in circuit with and normally electricallyisolating said first and second charge storing means, said second switchmeans characteriz/ed by having a predetermined switch-over voltage andat which its operational state is changed to electrically connect saidsecond charge storing means to said first charge storing means totransfer the charge on said second charge storing means to said firstcharge storing means, the switch-over voltage of said second switchmeans being below that of said first switch means.

8. The moisture sensing control of claim 7 wherein DC. voltage isapplied to said sensing means.

9. The moisture sensing control of claim 8 including voltage regulatingmeans in circuit with and controlling the voltage applied to saidsensing means to maintain a substantially uniform voltage thereon.

10. The moisture sensing controlof claim 8 including means in circuitwith and operative to vary the voltage applied to said sensing means.

11. The moisture sensing control of claim 8 including means operative toremove any residual charge from said first charge storing means prior toinitiation of operation of said drying apparatus.

12. A moisture sensing control for use with apparatus for varying themoisture content of material and comprising, in combination,

sensing means arranged in said apparatus for engagement with saidmaterial,

means for applying a voltage to said sensing means with the voltage onsaid sensing means varying in accordance with current leakage throughsaid material,

first switch means characterized by having a predetermined switch-overvoltage at which its operational state is change-d,

charge storing means in circuit with said first switch means and havinga charge storing capacity at least equal to the switch-over voltage ofsaid first switch means,

and transfer means responsive to the voltage on said sensing means andconnected in circuit with and normally electrically isolating saidsensing means and said first switch means, said transfer means operativeto periodically respond to a predetermined voltage on said sensing meansand electrically connect said sensing means to said first switch meansand apply said predetermined voltage to said first switch means, saidpredetermined voltage being below said switchover voltage so that saidcharge storing means is charged until the switch-over voltage of saidfirst switch means is reached whereupon the operational state of saidfirst switch means is changed to provide a control signal to effect acontrol function in said apparatus.

13. The moisture sensing control of claim 12 wherein said transfer meansincludes normally non-conducting switch means and time delay means incircuit between said sensing means and said normally non-conductingswitch means and operative to store a charge and provide a delay in theresponse of said normally non conducting switch means to the voltage onsaid sensing means and periodically rendering said non-conducting switchmeans conductive to transfer the charge from said time delay means tosaid charge storing means.

14. A moisture sensing control for use with apparatus for varying themoisture content of material and comprising, in combination,

sensing means arranged in said apparatus for engagement with saidmaterial,

means for applying a voltage to said sensing means with the voltage onsaid sensing means varying in accordance with current leakage throughsaid material,

first switch means characterized by having a predetermined switch-overvoltage at which its operational state is changed and operative toeffect a control function in said apparatus upon the change inoperational state thereof,

second normally non-conducting switch means in circuit with and normallyelectrically isolating said sensing means from said first switch meansand characterized by having a switch-over voltage below the switch-overvoltage of said first switch means so that said second switch means isresponsive to the voltage on said sensing means,

and electrical time delay means having a charge storing capacity atleast equal to the switch-over voltage of said first switch means andconnected in circuit be tween said second switch means and said firstswitch means for receiving and storing an electrical charge conductedthrough said second switch means until the charge accumulated is atleast equal to the switchover voltage of said first switch means.

15. The moisture sensing control of claim 14 including means in circuitwith said sensing means and said normally non-conducting switch meansfor providing a delay in the response of said normally non-conductingswitch means to the voltage on said sensing means.

16. The moisture sensing control of claim 15 wherein a DC. voltage isapplied to said sensing means.

17. A moisture sensing control for use with apparatus for dryingmaterial and operative to control the heat supply means of saidapparatus in accordance with the moisture content of said material, saidmoisture sensing control comprising, in combination,

sensing means adapted to have a voltage applied thereto and arranged forengagement with said material with the voltage on said sensing meansvarying in accordance with the current leakage through said material,

means for applying a DC voltage to said sensing means,

switch means connected with said heat supply means and having .aswitch-over voltage at which its operational state is changed to controlenergization of said heat supply means,

transfer means connected in circuit between and normally electricallyisolating said switch means from said sensing means, said transfer meansbeing responsive to the voltage on said sensing means to electricallyconnect said sensing means to said switch means, said transfer meansresponding to a voltage below said switch-over voltage,

and electrical timing means connected in circuit between said switchmeans and said transfer means for receiving and storing a charge untilthe accumulated charge thereon is at least equal to said switch-overvoltage so that a change in operational state of said switch means andde-energization of said heat supply means is effected with apredetermined time delay.

18. The moisture sensing control of claim 17 wherein said electricaltiming means comprises an RC time delay network.

19. A moisture sensing control for use with apparatus for dryingmaterial and operative to control the heat supply means thereof inaccordance with the moisture content of said material, said moisturesensing control comprising, in combination,

first switch means in circuit with and controlling said heat supplymeans,

relay means connected to and controlling said first switch means, secondswitch means connected to and controlling said relay means and having aswitch-over voltage at which its operational state is changed to changethe operational state of said relay means and de-energize said heatsupply means, sensing means adapted to have a voltage applied theretoand arranged for engagement with said material with the voltage on saidsensing means varying in accordance with the current leakage throughsaid material,

means for applying a DC. voltage to said sensing means,

transfer means responsive to the voltage on said sensing means andconnected in circuit between and normally electrically isolating saidsecond switch means from said sensing means, said transfer meansoperative to respond to a predetermined voltage on said sensing meansand electrically connect said sensing means to said second switch meansand apply said predetermined voltage to said second switch means, saidpredetermined Voltage being below said switch-over voltage,

and electrical timing means connected in circuit between said secondswitch means and said transfer means for receiving and storing a chargeuntil the accumulated charge thereon is at least equal to theswitch-over voltage of said second switch means so that a change in theoperational state of said second switch means and de-energization ofsaid heat supply means is effected with a predetermined time delay.

20. A moisture sensing control for use with apparatus for dryingmaterial and operative to control the heat supply means thereof inaccordance with the moisture content of said material, said moisturesensing control comprising, in combination,

first switch means in circuit with and controlling said heat supplymeans,

relay means connected to and controlling said first switch means,

second switch means connected to and controlling said relay means andhaving a switch-over voltage at which its operational state is changedto change the operational state of said relay means and de-energize saidheat supply means,

sensing means adapted to have a voltage applied thereto and arranged forengagement With said material with the voltage on said sensing meansvarying in accordance with the current leakage through said material,

means for applying a DC. voltage to said sensing means,

transfer means connected in circuit between and normally electricallyisolating said second switch means from said sensing means, saidtransfer means including normally non-conducting switch means having aswitch-over voltage below that of said second switch means and chargestoring means connected between said non-conducting switch means andsaid sensing means, said transfer means operative to periodicallytransfer an electrical charge from said sensing means to said secondswitch means with said charge being below said switch-over voltage,

and electrical timing means connected in circuit between said secondswitch means and said transfer means and including capacitive meansconnected between said second switch means and said non-conductingswitch means and having a charge storing capacity at least equal to theswitch-over voltage of said second switch means so that a change in theoperational state of said second switch means and deenergization of saidheat supply means is elfected with a predetermined time delay.

References Cited by the Examiner UNITED STATES PATENTS 4/1965 Chafee34-45 8/1965 Smith 3445

1. A MOISTURE SENSING CONTROL FOR USE WITH APPARATUS FOR DRYING MATERIALAND OPERATIVE TO CONTROL THE HEAT SUPPLY MEANS OF SAID APPARATUS INACCORDANCE WITH THE MOISTURE CONTENT OF SAID MATERIAL, SAID MOISTURESENSING CONTROL COMPRISING, IN COMBINATION, SENSING MEANS INCLUDING ANELECTRODE ARRANGED FOR ENGAGEMENT WITH SAID MATERIAL, MEANS FOR APPLYINGA D.C. VOLTAGE TO SAID ELECTRODE WITH THE VOLTAGE THEREON VARYING INACCORDANCE WITH CURRENT LEAKAGE THROUGH SAID MATERIAL SO THAT THEVOLTAGE ON SAID ELECTRODE INCREASES AS THE MOISTURE CONTENT OF SAIDMATERIAL DECREASES, SWITCH MEANS FOR CONTROLLING ENERGIZATION OF SAIDHEAT SUPPLY MEANS AND HAVING A SWITCH-OVER VOLTAGE AT WHICH ITSOPERATIONAL STATE IS CHANGED TO DE-ENERGIZE SAID HEAT SUPPLY MEANS, ANEON LAMP CONNECTED IN CIRCUIT BETWEEN SAID ELECTRODE AND SAID SWITCHMEANS AND HAVING A SWITCHOVER VOLTAGE AT WHICH IT IS RENDERED CONDUCTIVEAND BELOW WHICH IT IS NON-CONDUCTIVE, THE SWITCH-OVER VOLTAGE OF SAIDNEON LAMP BEING BELOW THAT OF SAID SWITCH MEANS AND THE VOLTAGE ON SAIDELECTRODE AT THE INITIATION OF THE DRYING OPERATION BEING BELOW THESWITCH-OVER VOLTAGE OF SAID NEON LAMP SO THAT SAID SWITCH MEANS ISELECTRICALLY ISOLATED FROM SAID ELECTRODE AND IS IN A FIRST OPERATIONALSTATE, CAPACITIVE MEANS CONNECTED IN CIRCUIT WITH SAID NEON LAMP ANDELECTRODE FOR RECEIVING AND STORING A CHARGE FROM SAID ELECTRODE ANDCHARACTERIZED BY HAVING A CAPACITY AT LEAST EQUAL TO THE SWITCH-OVERVOLTAGE OF SAID NEON LAMP, AND CAPACITIVE MEANS CONNECTED IN CIRCUITBETWEEN SAID NEON LAMP AND SAID SWITCH MEANS AND CHARACTERIZED BY HAVINGA CAPACITY AT LEAST EQUAL TO THE SWITCHOVER VOLTAGE OF SAID SWITCHMEANS.